Method of making steel backed bronze bearings



I Patented Dec. 24, 1940 METHOD OF MAKING STEEL BACKED BRONZE names Louis Sandler and Louis G. Klinker, New Castle, Pa., assignors to Johnson Bronze Company, New Castle, Pa., a corporation of Pennsylvania No Drawing. Application May 22, 1939, Serial No. 275,005

7 Claims.

The present invention relates to steel backed bronze bearings and the method of making, and more especially to a steel backed lead-containing bronze bearing made by a sintered powder process.

Heretofore steel backed bronze bearings have usually been made by flowing a thin layer of molten bronze onto a steel backing strip. The

, bronze facing is then milled down to the de- 10 sired thinness. The strip is then rolled and afterwards cut up into sheets and bent into circular form to form the bearings. This vrequires a rather delicate and expensive casting operation together with the milling operation. The bronze l is solid and not adapted for so-called oill'ess bearings.

In accordance with the present invention the bronze in powder form is applied to a steel backing strip and is sintered thereto. The bronze 2 facing can be sized solely by rolling operations which are much more economical than milling. A very thin coating of bronze may be applied to the steel further conducing to economy. Solid lubricants, such as graphite may be incorporated in the bronze. Also, the bearing may be given any desire-d porosity so that it may be impreg nated with oil.

An important feature of our bearing is the abilityto make a bronze sheet shaped by a roll- 3 ing process containing any desired amount of lead. While lead may be incorporated in cast bronzes up to 35%, high leaded bronzes cannot be rolled. If more than about 4% of lead is incorporated in the bronze billet, when the billet 35 is rolled the lead particles which are of a considerable volume if the lead content is high are elongated and cause lines of weakness in the metal so that it cracks readily along the axes of the lead particles. Inour process of rolling, 40 the bronze is primarily compressed rather than elongatedso that a high percentage of lead may be incorporated in the metal.

In carrying out our invention, a strip of steel is prepared to receive a coating of powdered 45 bronze. The surface must be cleaned thoroughly and a better bond is obtained when a zinc chloride solution is applied to the surface and thoroughly dried before the powdered bronze is applied. Any other flux of a similar naturecan 50 be used. A plain strip of steel may be used, or if it is desired it may be tin plated or copper plated.

A layer of powdered bronze is then spread on the strip. The process is preferably carried out 65 continuously and the bronze powder applied as the strip is passed through suitable mechanism for applying and smoothing outthe layer of bronze powder. The powder mayconsist of fine- 1! ground bronze particles alone or the bronze particles may be 'copperized as hereinafter de 5 scribed, or they may have mixed with them minor amounts of powdered copper, tin, zinc, alumimm, or other metal, to assist in the sintering operation. The bronze powder should be of a size which will passthrough an 80-mesh sieve 10.

since the finer the powder the finer the grain of the material produced. It is not, however,

economical to grind the bronze to pass through a smaller than a 200-mesh sieve.

. The bronze powder is spread uniformly on the steel strip by any suitable mechanism which will smooth it out to the desired thickness, preferably about .045 to .055 inch thick. *The strip. is then passed through-a hydrogen atmosphere sintering furnace at a temperature of about 1250" F. to 1500 F. This causes the particles of bronze powder to become sintered together and onto the v steel backing. Theatmosphere in the sintering furnace should be non-oxidizing, either neutral r or reducing. Since "the usual methods of grlnding leave the bronze particles with oxide films on them, a reducing-atmosphere is preferably employed. It has been found that'the presence of the oxide films on the bronze particles, which films are reduced in the sintering operation, tends 80 to cause better sintering and adhesion between the bronze particles probably because of the high activity of the nascent metal resulting from the reduction of the oxide films. The steeljstrip,

after coming from the furnace, is run through an a cooling chamber having a hydrogen atmosphere to prevent oxidation during cooling.-The sintered bronze adheres to the steel backing with a fair amount of tenacity.

The composite strip is then cold rolled whereby the sintered bronze layer is reduced to a thickness of approximately .025 inch.- The rolled strip is' then put through another furnace having a non-oxidizing, preferably hydrogen, atmosphere at a temperature of between 1250 and 1500? 'F. Q

The layer of bronze which has been consolidated and rolled into close contact with the steel strip in the preceding rolling operation is'bonded' to the backing with a bond which appears to be substantially perfect under the microscope, so that the composite strip can .be bent and fractured without breaking the bronze layer loose from the steel backing.

After this second heating the composite strip is sized by another rolling operation which slightly 5 reduces the thickness of the bronze sheet by .005 inch or more, depending upon the desired hardness of the bronze strip. The greater the consolidation at this point the harder the strip. The second heating operation tends to anneal the bronze strip and soften it but the second rolling operation mechanically works the bronze to re-harden it.

After the composite strip has been re-rolled, it is ready for use for a bearing material and can be cut and stamped.

The bronze powder is preferably formed by grinding a lead-containing bearing bronze which may contain lead in the usual percentages for cast bearing bronzes. When the bronze is cast from molten condition the lead which separates from the bronze is distributed through the bronze metal as microscopic inclusions of metallic lead. When the bronze is ground to a powder the original crystalline structure of the bronze with the metallic lead inclusions is retained. Since the bronze powder is spread in a relatively thin layer on the horizontal sheet of steel there is little if any tendency for the lead to run out even with very high lead content in the bronze from which the powder has been ground.

The rolling operations to which the composite sheet is subjected, are primarily consolidating operations and serve to compress the porous sintered mass so that there is but little elongation. The lead particles are retained in the finished product in substantially their original shape so that lines of weakness are not formed as is the case when it is attempted to roll bronzes of high lead content into sheets, so this makes it possible to incorporate practically any desired percentage of lead in the bronze.

The sizing of the sheet is accomplished entirely by the rolling operations which are rfiuch less expensive than the milling operations required to size a composite sheet consisting of a backing of bronze cast against it. Our process lends itself particularly to the production of so-called oilless bearings. Graphite or other similar solid lubricating material may be mixed with the bronze powder. The porosity of the bronze may be controlled by the degree of compression in the rolling operations. The bronze may be rolled to a high density in which case a hard bearing is produced, or by lighter rolling a considerable porosity may be left in the material so that it may be impregnated by dipping in oil. This, with the addition of graphite, makes an excellent so-called oilless bearing.

Oilless porous bearings have an inherently weak structure, the greater the porosity the weaker the structure. However, the porous bronze has been sintered into intimate contact with the steel backing, which gives the composite bearing a high strength, despite the porosity of the bronze facing.

The copperizing process above referred to, to condition the ground particles of bronze is carried out as follows, and has for its object the roughening of the smooth surfaces of the bronze particles: When bronze is ground round or nodular particles are formed which have smooth surfaces. These surfaces may be roughened by mixing the bronze powder with'a smaller amount of copper powder the particles of which are preferably somewhat smaller than those of the bronze powder. The mixture is heated in a reducing atmosphere to cause the copper particles to become sintered to the bronze particles and copperize them. The sintering temperature is preferably in the neighborhood of 1300 F. It should be below the fusing point of the bronze or the copper, but high enough to cause sintering of the copper particles to the smooth surface of the bronze particles. The sintered on copper particles act as sort of tentacles and assist in the interlocking of the bronze particles during the sintering and rolling operations.

Instead of copperizing the bronze particles, small amounts of copper and tin powder preferably of somewhat finer particle size than the bronze particles are mixed with the bronze powder. During the sintering operation the copper and tin particles tend to become fused together and to form a matrix for joining and surrounding the bronze particles. While it is preferred to either copperize the bronze particles or mix with them a small amount of copper and tin powder. the bronze powder alone may be used.

When the copper and tin are melted together a perfect mixture takes place. When the molten metal solidifies, the crystalline structure characteristic of cast bronze is produced. In most bearing bronzes suflicient tin is employed so that the solidified product contains not only the alpha solid solution of tin and copper, but also the socalled alpha-delta eutectoid which contains the higher tin delta constituent of cast bronze which imparts the desired hardness to the bronze. By using a powder ground from a pre-cast bronze, the crystalline characteristics of the cast bronze are maintained in the sintered bearing and a product is secured which is superior to that made by sintering together copper and tin powder. Furthermore, since the bronze has solidified from molten condition, it does not have the tendency to the objectionable growth encountered in sintered mixtures of copper and tin.

So far as we are aware, the only known way of getting a lead-bearing bronze with the lead homogeneously distributed in it is by casting, and therefore the only way to get lead-bearing bronze particles in which the lead is homogeneously distributed among the grains of bronze is by grinding up a previously cast bronze in which the lead is thoroughly distributed. By using a bronze powder ground from a lead-bearing bronze, a homogeneous distribution of the lead in the bronze is obtained, and since the rolling operations result in compacting rather than elongating the material, the lead is retained in the form of fine nodular inclusions.

By our method a very thin facing of bronze may be applied to the steel backing. If a bearing becomes worn more than a few thousandths of an inch, it is discarded, so that only a thin facing of bronze is necessary in the finished bearing. By our process we are enabled to apply as thin a facing as is desired without any waste.

While We have described the preferred embodiments and practice of our invention, it is to be distinctly understood that the invention may be otherwise embodied and practiced within the scope of the following claims.

We claim:

1. The method of making bearings which comprises applying to a steel backing a layer of powdered previously cast bronze, heating the backing and applied powder while still uncom-- pacted in a non-oxidizing atmosphere to sinter the powder and cause it to bond with the steel backing, and subjecting the composite sheet to a consolidating operation.

2. The method of making steel backed bronze bearings which comprises applying a layer of powdered previously cast bronze to a steel backing, heating the backing and applied layer of bronze to cause the bronze 'to become sintered and united to the steel backing, rolling the composite sheet, re-heating the rolled sheet to further unite the bronze to the steel backing, and re-rolling the composite sheet.

3. The method of making steel backed leadcontaining bronze bearings which comprises grinding a lead-containing cast bronze to a fine powder, applying a layer of the powder to a steel backing, heating the backing and applied bronze powder while still uncompacted in a non-oxidiz ing atmosphere to sinter the powder and cause it to adhere to the steel backing, and consolidating the composite sheet by rolling it but without substantial elongation.

, 4. The method of making steel backed leadcontaining bronze bearings which comprises grinding a lead-containing cast bronze to a fine powder, applying a layer of the powder to a steel heating the composite sheet'and re-rolling it.

. '5. The method of making bearings which comprises grinding a lead-containing cast bronze to a so as to sinter the particles or copper powder to the bronze particles, applying a layer of the thus copperized bronze particles to a steel backing, heating the backing and applied bronze powder in a non-oxidizing atmosphere to sinter the powder and cause it to adhere to the steel backing, and consolidating the composite sheet by rolling.

6. The method of making steel backed leadphere to sinter the powder and cause it to adhere to the steel backing, and consolidating the composite sheet by rolling.

7. The method of making steel backed leadcontaining bronze bearings which comprises grinding a lead-containing cast bronze to a fine powder, applying a layer of the powder to'a steel backing, heating the backing and applied bronze" powder while still uncompacted in a non-oxidizio v ing atmosphere to sinter the powder and cause it to adhere to the steel backing, and thereafter rolling the composite sheet to compact it; 

